The Universe is commonly defined as the totality of existence, [ 1 ] [ 2 ] [ 3 ] [ 4 ] including planets , stars , galaxies , the contents of intergalactic space , and all matter and energy . [ 5 ] [ 6 ] The broadest definition of universe is that it is simply everything, while a narrower definition is that the universe is limited to what can be observed. [ dubious ] Similar terms include the cosmos , the world and nature . Scientific observation of the Universe, the observable part of which is about 93 billion light years in diameter, [ 7 ] has led to inferences of its earlier stages. These observations suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history.
Educational video on supernovae explosions from NASA SN 2007ck and SN 2007co in the same galaxy A supernova (abbreviated SN , plural SN e after "supernovae") is a stellar explosion that is more energetic than a nova .
A star is a massive, luminous sphere of plasma held together by gravity . The nearest star to Earth is the Sun , which is the source of most of the energy on the planet. Some other stars are visible from Earth during the night when they are not obscured by atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance.
The Solar System [ a ] consists of the Sun and its planetary system of eight planets , their moons , and other non-stellar objects .
A planet (from Ancient Greek αστήρ πλανήτης (astēr planētēs) , meaning "wandering star") is an astronomical object orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity , is not massive enough to cause thermonuclear fusion , and has cleared its neighbouring region of planetesimals . [ 1 ] [ 2 ] The term planet is ancient, with ties to history , science , mythology , and religion . The planets were originally seen by many early cultures as divine, or as emissaries of deities . As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects.
A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. [ 1 ] These quantities are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. For the purposes of proving the Penrose–Hawking singularity theorems , a spacetime with a singularity is defined to be one that contains geodesics that cannot be extended in a smooth manner. [ 2 ] The end of such a geodesic is considered to be the singularity.
In physics , gravitational waves are ripples in the curvature of spacetime which propagate as a wave , travelling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity , [ 1 ] gravitational waves theoretically transport energy as gravitational radiation . Sources of detectable gravitational waves could possibly include binary star systems composed of white dwarfs , neutron stars , or black holes .
Gravitation , or gravity , is the natural phenomenon by which physical bodies appear to attract each other with a force proportional to their masses .
A galaxy is a massive, gravitationally bound system consisting of stars , stellar remnants , and interstellar medium of gas and dust , and, it is hypothesized, an important but poorly understood component called dark matter . [ 1 ] [ 2 ] The word galaxy is derived from the Greek galaxias ( γαλαξίας ), literally "milky", a reference to the Milky Way . Examples of galaxies range from dwarfs with as few as ten million (10 7 ) stars [ 3 ] to giants with a hundred trillion (10 14 ) stars, [ 4 ] each orbiting their galaxy's own center of mass .
Estimated distribution of matter and energy in the universe, today (top) and when the CMB was released (bottom)
In physical cosmology and astronomy , dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe . [ 1 ] Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is expanding at an accelerating rate . In the standard model of cosmology , dark energy currently accounts for 68.3% of the total mass–energy of the universe. [ 2 ] Two proposed forms for dark energy are the cosmological constant , a constant energy density filling space homogeneously, [ 3 ] and scalar fields such as quintessence or moduli , dynamic quantities whose energy density can vary in time and space.
In the general sense, a cosmos is an orderly or harmonious system . The word derives from the Greek term κόσμος ( kosmos ), literally meaning "order" or "ornament" and metaphorically "world", [ 1 ] and is antithetical to the concept of chaos . Today, the word is generally used as a synonym of the Latin loanword " Universe " (considered in its orderly aspect).
Cosmic strings are hypothetical 1-dimensional (spatially) topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking is not simply connected .
Temperature of the cosmic background radiation spectrum as determined with the COBE satellite: uncorrected (top), corrected for the dipole term due to our peculiar velocity (middle), and corrected for contributions from the dipole term and from our galaxy (bottom).
A black hole is a region of spacetime from which gravity prevents anything, including light , from escaping. [ 1 ] The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.